Slush generation machines that provide a partially frozen liquid are well known. Such machines are often used to create a semi-frozen slush that can be used when a high thermal capacity fluid coolant is needed to provide either a high rate of cooling or consistent cooling for long periods. Typically, the slush is formed from water with a freeze point suppressant—common examples of which include sugars, salts, alcohols and glycols—which decreases the freeze temperature and stops ice crystals joining together and blocking the machine.
Slush machines are used in the medical industry when saline slush is used to cool tissue to reduce metabolic rate to reduce damage during surgical procedures and in sports therapy to accelerate healing of injuries. Of the slush machines used to create slush for surgical procedures many create the small ice crystals needed to make the semi-liquid slush either by freezing the working fluid on a refrigerated surface and then removing it using a mechanical scraper or by mechanically flexing the surface. Such systems are generally expensive, requiring large geared motors to drive the scrapers or surface flexing mechanisms. The key requirement of a slush fluid for use in medical procedures, particularly when the slush is used internally, is that the fluid is sterile to prevent infection. Scraped surface slush machines are unsuited to this application as the freezer system is fundamentally expensive and so must be a reusable component that must be regularly sterilised. US007874167B2 describes a machine for creating slush for surgical use. Other applications of slush include food processing and cold storage for air conditioning.
Slush generation machines that provide a partially frozen liquid beverage are also well known. Unlike drinks containing water ice cubes, they provide beverages in which the beverage itself is frozen and used to maintain a low drink temperature throughout consumption. This has the advantage of maintaining the concentration of the flavouring ingredients of the beverage and not ‘watering down’ the drink. In addition, the texture of the ice crystals in the drink can provide a desirable ‘mouth feel’ and enhanced experience for the consumer. Two types of system for the production of slush beverages are known.
In the first type of system, liquid beverage dwells in a refrigerated chamber and turns to ice on the refrigerated surface of the chamber. This ice is dislodged by means of a scraper to mix with the liquid and form a slush. Such scraped surface freezer systems are disclosed e.g. in WO2009/037446, WO2009/060169, EP1738652, WO2004/088220, U.S. Pat. No. 3,823,571 and EP2446750. These systems tend to be very expensive due to the large geared motors needed to drive the scrapers. In addition, particularly when pressurised, these systems are unreliable as the rotating seals needed to transmit the mechanical power are prone to failure. Other drawbacks to these systems include high energy input due to the use of vapour compression refrigeration systems to directly cool the freezer surfaces. To achieve sufficient cooling power to provide the required throughput of slush beverages the refrigeration plants must be large, low temperature, systems giving low coefficients of performance and high energy input. The cost of product wastage when the systems are cleaned, particularly when the beverage is expensive such as with alcoholic drinks, can also be a problem. With scraped surface ice generators the typical holding volume is several liters, all of which will be discarded when the system is cleaned.
In the second type of system, liquid beverage does not dwell but instead recirculates through at least one long coil that is suspended in a glycol cooling medium. Such systems are disclosed in US2001/0041210 and WO2011/051707. Ice forms in the liquid, resulting in a slush, the temperature of the cooling medium being chosen to achieve the desired ice/liquid ratio in the slush—WO2011/051707 discloses a temperature of −5.6° C. corresponding to an estimated 20% ice/liquid ratio and a temperature of −6.4° C. corresponding to 24% ice/liquid ratio. This temperature must be limited so as to avoid complete freezing of the slush in the conduit when throughput is low and little or no unfrozen liquid is entering to replace dispensed partially frozen liquid. To achieve the freeze rate necessary for when the throughput is higher, the length of the conduit must be increased to around 72 m in length, making the apparatus as a whole less compact. In addition, the system uses one or more variable speed pumps for the beverage and an integrated refrigerating system with a glycol bath. As a result, it is inherently large, complicated and expensive.